1. Field of the Invention
The present invention relates to an optical head. More particularly, the present invention relates to a micro optical head capable of providing a sub-wavelength light spot and has an enough depth of focus.
2. Description of Related Art
To converge light beams smaller is a pursuing object for physicists. However, when a light beam is converged, the depth of focus is shortened, or the transmission energy is dramatically decreased, which greatly limits the development of photolithography and optical storage.
The size of a focusing spot of an optical lens is mainly restricted by the diffraction limit. As the wave characteristics of light include interference and diffraction, in a far field range, the size of the focusing spot of a lens is determined by the wavelength of incident light as well as the numerical aperture (NA) of the lens used. The depth of focus (DOF) of the lens is also affected by the wavelength and the NA. Theoretically, the size of the focusing spot cannot be smaller than 0.61λ (λ is the wavelength of the incident light). Moreover, when the focusing spot is shrunk, the DOF is also reduced. In order to obtain a smaller light spot, a lens with a greater NA can be used in addition to shortening the wavelength. However, lens with larger NA will further reduces the DOF, and therefore, the requirements on the environment and the precision of platform control during exposure and etching become even stricter.
Currently, the known method for overcoming the diffraction limit is to utilize the principles of near field optics. That is, the exposure is performed within several tens of nanometers where no obvious diffraction occurs after light passes through a nanometer-scale optical hole, so as to obtain a light spot equivalent to the size of the hole. However, when the light passes through a hole smaller than its wavelength, the transmission is relative to the fourth power of the ratio of hole diameter to the wavelength ((d/λ)4). It is known from the above that in order to obtain a sub-wavelength spot with the near field optical method, one of the limits is the intensity of the energy transmitting through the hole. The diffraction will be more serious for a smaller hole, and the size of the light spot will expand drastically when the distance to the hole becomes longer. Therefore, the distance of the sub-wavelength light spot to the hole is within several tens of nanometers, and the working distance must be controlled by a precise feedback mechanism.
In addition, Ebbesen et al. reported phenomenon of extraordinary optical transmission in 1998. That is, for incident light at specific wavelengths, the transmission through a silver film is extraordinarily increased. The wavelengths leading to the extraordinary transmission are about ten times greater than the diameter of the hole. The wavelengths leading to the extraordinary transmission are relative to the period and arrangement of the hole array, and the intensity of the transmission energy is relative to the depth-to-width ratio of the holes. Another key factor is the material of the metal film, and the extraordinary transmission phenomenon is only occurred for some specific metals.
In addition to metal sub-wavelength structures that may realize light beams with a smaller focusing spot and a longer DOF, a light beam referred to as Bessel beam also has the characteristics of non-divergence, that is, the DOF is infinite in theory. Scientists are trying different-experimental configuration to generate the Bessel beams. For example, a laser irradiates onto an annular mask placed at front focal plane of a lens, so as to form a Bessel beam in the region behind the lens. Alternatively, a laser irradiates onto a conical lens (Axicon) or a holographic element, will also generate a Bessel beam in the region behind the lens. However, the elements applied in the above methods still have sizes of conventional optical elements. Though micron-scale axicon are fabricated by E-beam lithography recently, the Bessel beam is still generated according to the traditional principle. For example, it is proposed that the annular mask is placed at the front focal plane of the lens to generate the Bessel beam without divergence. However, in actual implementations, a lens must be added behind the annular aperture, so the entire opto-mechanical system is hard to be miniaturized. Besides, other researchers use a single annular aperture as a mask, and the light passing through the mask interferes with another reference Gaussian beam to generate the Bessel beam. However, till now, the elements used in above methods are in the size of conventional optical elements (mm-cm).
As the conventional focusing lens and near field optical methods both have limitations in actual application, it is required to provide an optical head that is easy to fabricate and suitable for being miniaturized, and can generate a sub-wavelength focusing light spot with a very long DOF.